Saying there isn't an instance where A outperforms B is NOT the same as saying B is worthless to the party on a whole. I'm sure a RNG with the best of everything can outdamage any other character, so does that mean MNK (on non-undead) and DRK are worthless to every party? The fact is I proved that there exists conditions where your fact is not true, which means your fact isn't a fact. Accept it and move on.
No. No no no. The point, using NIN/THF as the example, is that NIN's
only strong point is tanking. If he isnt tanking, he isnt doing anything else well enough to warrent an invite. He will slow you down, and if he doesnt slow you down, you still get a extra member divided into your xp.
Would you invite a MNK and use him as a ranged attacker? Of course not. He is a melee damage dealer.
How about a DRK to use for buffs and support? No. He is a melee damage dealer.
Would you invite a NIN and use him as a damage dealer? No way. He is an evasion tank.
Comparing NIN/THF (A job combo that cannot perform the one thing it excells at) with a MNK or a DRK is borderline retarded. They may not deal as much damage as a RNG, but they have other advantages that RNGs do not (if you dont know these, just ask). NIN as a damage dealer doesnt bring anything to the table, alongside its piss-poor damage.
My 6th-worthless member examples only work if your 6th member is in fact completely crappy. MNKs and DRKs trying to deal melee damage are anything BUT crappy. NINs trying to deal melee damage are laughable. Why not invite a SMN/SAM that uses bloodpacts and staff skills to Skillchain with himself (after the CoP patch)? That would be just as useful as a NIN/THF, and while adding nothing, would still take a cut of the xp.
Dont be so naive as to think that you can magically make NIN/THF non worthless by spamming shuricans (a feat which is always better as NIN/RNG). This game isnt built like that. You cant just take whatever combo you want and make up a reason why it might not suck in one or two situations. NIN/THF has one use. Pulling. But if I wanted a puller, I could probably get one that aided the party farther after the pull, rather than giving a chunk of my xp to a guy who just pulls mobs back to me.
I dont really know what else to say. If you dont buy into my statements about 5 being better than 5+0 (your extra member), then by all means.. invite a fill in on your 5 man group that wont contribute. He can have some of your xp, and you can move at close to the same speed as you would have without him. Your choice.
Moral: In FFXI, if you are not contributing to the group, you are hurting it. Any extra speed you might gain with the shit damage of a NIN or PLD damage dealer is negated by the extra person being divided into the xp you get.